Why do some public organizations survive for many decades, whereas others are terminated within a few years? This question of organizational survival has long intrigued public administration scholars. To explain longevity, public administration research has focused on organizational design features and adaptive capacities. The results have been inconclusive. This article explores an additional explanation for survival and demise: the density dependence theory as formulated in the field of organizational ecology. The underlying premise of this theory is that certain environments can only sustain a certain number of similar-type organizations. A rising number of organizations fuels competition for scarce resources, which inevitably leads to the demise of organizations. Density theory has often been tested in the business literature, but has been rarely applied to public sector organizations. In this article, we test whether this theory can help explain organizational survival in a population of US federal independent public agencies (n = 142). Our results show that density matters. This is good news for public administration research: the inclusion of density boosts the explanatory power of traditional variables such as design and adaptation.